Part of what makes Inception's ending great is that it's open to interpretation.  In an interview with the BBC's Chris Moyles, Michael Caine gave his opinion on the film's ambiguous final shot.  For those who haven't seen the film, I've put Caine's thoughts on the matter after the jump.  Moyles also spoke to Caine about Batman 3 and you can read what Caine had to say on that film by clicking here.

After the image below, you can read what Caine said about the ending to Inception [via AICN]: [Spoiler, obviously]:

Caine says that Cobb is no longer in a dream and that the final scene is real. (i.e. the spinning top falls)

While that ending is happier, I think it's also kind of dull.  I find the arguments that the entire film is a dream to be far more fascinating.  Some may argue that there is an objective truth to what the ending actually is, but Nolan clearly left it ambiguous in order to spark discussion.  Saying that there is only one answer closes down that discussion and makes the film less interesting as a result.